FIG. 9 is a block diagram showing a structure of a conventional multimode optical transmission system. In FIG. 9, the conventional multimode optical transmission system comprises a laser diode 901, a photo diode 902 and a multimode optical transmission path 903. The laser diode 901 converts an inputted electrical signal into an optical signal, and then outputs the optical signal to the multimode optical transmission path 903. The multimode optical transmission path 903 transmits the optical signal outputted from the laser diode 901 to the photo diode 902. The photo diode 902 converts the inputted optical signal into an electrical signal.
In more detail, a multimode optical fiber is used as the multimode optical transmission path 903. The optical signal having a single wavelength λ, which is outputted from the laser diode 901, is inputted into the multimode optical fiber. Since a core diameter of the multimode optical fiber is greater than that of a single mode optical fiber, there exist a plurality of propagation paths of the optical signal within the multimode optical fiber. In general, a multimode optical fiber has a core diameter of approximately 50 μm, whereas a single mode optical fiber has a core diameter of approximately 10 μm.
Here, each of optical signals respectively having different propagation paths is referred to as a mode. A mode having a smallest angle of incidence on an optical fiber is a fundamental mode. A transmission distance of an optical signal is shortest when the optical signal is in the fundamental mode. As an order of the mode becomes greater, the angle of incidence on the optical fiber also becomes greater, and a propagation distance of the optical signal becomes longer. Assuming that a longitudinal direction of the optical fiber is a Z-axis, a Z-axis component of a wave number k is referred to as a propagation constant β, and an equation β=k cos φ is satisfied. Here, an angle of the optical signal with respect to the Z-axis is φ. Accordingly, each mode of an optical signal has a different propagation constant, and the fundamental mode has a largest propagation constant. Here, an optical signal having all modes is inputted into the photo diode 902. The photo diode 902 converts the inputted optical signal having all the modes into an electrical signal.
In a system using the multimode optical fiber (i.e., the multimode optical transmission system), since the core diameter of the multimode optical fiber is large, a highly precise connection is not required between the multimode optical fiber and each of peripheral parts such as the laser diode 901 and the photo diode 902. As a result, the multimode optical transmission system can be constructed at a lower cost compared with a system using a the single mode optical fiber (i.e., a single mode optical transmission system). For this reason, the multimode optical transmission system is currently in widespread use for a system such as an office LAN for transmitting an optical signal within a relatively short distance (refer to a non-patent document 1).
[Non-Patent Document 1] Tetsuya Miki, et al., “Handbook of Optical Communication Technology”, The Optronics Co., Ltd., pp. 199 to pp. 201, 2002 (ISBN 4-900474-91-6).
[Non-Patent Document 2] Katsunari Okamoto, “Fundamentals of Optical Waveguides”, Corona Publishing Co., Ltd., p 83, FIG. 3.12, 1992 (ISBN 4-339-00602-5).